North Korea has rescinded an invitation for a U.S. envoy to visit North Korea and try to secure the release of a detained American.

Ambassador Robert King, President Barack Obama's special envoy for North Korean human rights issues, was expected to fly to the Asian nation on Friday to try to win the freedom of Kenneth Bae, an American citizen and University of Oregon graduate, imprisoned there for carrying out "serious crimes" against Kim Jong Un's regime.

North Korean authorities detained Bae, a Christian missionary, last year and sentenced him to 15 years of hard labor earlier this year. They said he had planned an operation to bring down the government through religious activities.

Bae's family has said he was the owner of a tour company who was in North Korea for work.

His sister, Terri Chung, said earlier this month that Bae's health has severely deteriorated during his imprisonment and that he has been transferred to a hospital. He has spent more time in North Korean custody than any other American.

King was to travel to Pyongyang at North Korea's invitation, the U.S. State Department said this week.

The envoy, who has been traveling in the region, was to join a small delegation flying to North Korea's capital on an American military jet on Friday, a U.S. official told CNN this week.

King led a U.S. delegation that in 2011 secured the release of Eddie Yong Su Jun, a Korean-American businessman who was detained in North Korea for several months.

Calls for clemency

The North Korean supreme court in April found Bae guilty of carrying out "serious crimes" against the government, including setting up bases in China for the purpose of toppling Kim's regime, encouraging North Korean citizens to bring down the government and conducting a smear campaign, according to the country's state media.

King was to ask Pyongyang to pardon Bae and grant him amnesty on humanitarian grounds "so that he can be reunited with his family and seek medical treatment," the State Department said.

The White House, in a separate statement, had urged the government of North Korea "to grant special clemency to Mr. Bae immediately and allow him to return home with Ambassador King."

Bae suffers from severe back and leg pain and has lost more than 50 pounds, Chung said.

She said she received the information from the State Department, which told her the Swedish ambassador visited Bae in the hospital. Sweden represents U.S. interests in North Korea because the United States has no diplomatic presence there.

Bae also suffers from kidney stones, dizziness, blurred vision and loss of vision, Chung said. He was already dealing with other health problems, including diabetes.

Tensions between North Korea and the United States have eased somewhat since the spring, when Pyongyang unleashed a torrent of dramatic threats as U.S. and South Korean troops carried out large-scale military exercises in the region.

The uneasy period followed tougher U.N. sanctions on North Korea after the reclusive state carried out a long-range rocket launch in December and an underground nuclear test in February.